This invention relates to a cryogenic vessel for a superconducting apparatus, and more particularly but not exclusively, it relates to a cryogenic vessel for cooling the superconducting magnet of a superconducting apparatus such as a nuclear magnetic resonance diagnosis apparatus.
A conventional cryogenic vessel which was disclosed in Japanese Patent Publication No. 59-17550 is illustrated in FIG. 1. In that invention, a cylindrical, substantially horizontal inner vessel 3 which contains a superconducting magnet 1 and a freezing mixture 2 of liquid helium or the like is contained with a vacuum insulating vessel 4 which thermally insulates the inner vessel 3. A number of heat shields 5 (only one of which is shown in the figure) which are at an intermediate temperature are disposed between the inner vessel 3 and the vacuum insulating vessel 4 and serve to insulate the inner vessel 3. The inner vessel 3 is supported by support members 6 whose outer ends are secured to the vacuum insulating vessel 4. The inside of the inner vessel 3 is connected with the outside of the vacuum insulating vessel 4 by a substantially horizontal connecting pipe 7. While the inner end of the connecting pipe 7 opens onto the inside of the inner vessel 3, the outer end is sealed by a rupture-type safety valve 8. The safety valve 8 contains a thin plate which ruptures when the pressure inside the connecting pipe 7 exceeds a certain value. As shown in FIG. 2, a plurality of disk-shaped dividing plates 9 made of a material with a low thermal conductivity are disposed at intervals on the inside of the connecting pipe 7. These dividing plates 9 divide the connecting pipe 7 into a plurality of small compartments and thereby reduce convection by gas contained within the connecting pipe 7. Each of these dividing plates 9 is designed so as to rupture or open at or below the pressure at which the safety valve 8 ruptures so that when the gas pressure within the inner vessel 3 exceeds a certain value, the gas can be discharged through the connecting pipe 7.
Although the dividing plates 9 are effective in reducing the penetration of heat into the inner vessel 3 due to convection by gas within the connecting pipe 7, they can not prevent the penetration of heat by conduction along the walls of the connecting pipe 7, and a large amount of heat can enter the inner vessel 3 in this manner. Accordingly, the illustrated cryogenic vessel is not entirely satisfactory with respect to its thermal insulation.
Another problem with the illustrated conventional apparatus is that the dimensions can not be reduced without increasing the penetration of heat along the connecting pipe 7. Namely, since the connecting pipe 7 connects to the outer surface of the inner vessel 3, if the external dimensions of the entire vessel are to be decreased by decreasing the size of the spaces between the inner vessel 3, the heat shield 5, and the vacuum insulating vessel as far as possible while still enabling assembly, the length of the connecting pipe 7 is inevitably decreased, causing an increase in the conduction of heat along the connecting pipe 7 and increasing the consumption of the cryogenic mixture 2.
FIG. 3 illustrates a portion of another conventional cryogenic vessel as disclosed in Japanese Patent Publication No. 54-53359. As shown in this figure, a superconducting magnet 1 is electrically connected to a power supply 12 by power supply leads 11 which extend from the outside of a vacuum insulating vessel 4 to the inside of an inner vessel 3 filled with a cryogenic mixture 2 such as liquid helium. The power supply leads 11 are supported by a wiring support pipe 13 whose outer end is secured to the vacuum insulating vessel 4 and whose inner end is secured to the inner vessel 3. The wiring suppport pipe 13 is divided into an upper and lower portion which are connected with one another by a flexible tube 14. The flexible tube 14 enables the upper and lower portion to move with respect to one another when thermal shrinkage occurs due to the cooling of the various parts of the vessel, thereby preventing thermal stresses from developing in the wiring support pipe 13. The upper ends of the power supply leads 11 are secured to the wiring support pipe 13 by a flange 15 mounted on its upper end, and the lower ends of the leads 11 are secured to the wiring support pipe 13 by a connector 16 which is mounted on the inner end thereof and which electrically connects the power supply leads 11 to the superconducting magnet 1. As in the vessel of FIG. 1, the inner vessel 3 is supported by the vacuum insulating vessel 4 through support members 10.
While the flexible tube 14 is effective to prevent thermal stresses from developing in the wiring support tube 13, it results in thermal stresses arising in the power supply leads 11, since the change in dimensions of the power supply leads 11 due to thermal shrinkage is much less than that of the wiring support pipe 13 and the flexible tube 14. For this reason, it is very difficult to reliably connect the power supply leads 11 to the connector 16.